British Reaper drones (above) will be used alongside Rivet Joint planes in Syria. Photograph: Ministry of Defence/EPA

Britain is to send military drones over Syria to gather intelligence in a move that will deepen its involvement in the campaign against Islamic State (Isis), Michael Fallon, the defence secretary, has revealed.

Downing Street insisted that the flights did not amount to military intervention and said there was a clear legal case for drone surveillance in Syria under the principles of “national and collective defence”.

The Reaper drones have already been active in Iraq, after parliament gave its approval for Britain to take part in air strikes against Isis. However, this will be the first time they will have ventured into Syrian territory, where David Cameron has not sought approval for military action because of fears it would be blocked by Labour and some within the prime minister’s own party.

In a written ministerial statement, Fallon said Reaper drones would be starting operations “very shortly” and would be used alongside Rivet Joint planes.

“As well as their operations over Iraq, both Reapers and Rivet Joint surveillance aircraft will be authorised to fly surveillance missions over Syria to gather intelligence as part of our efforts to protect our national security from the terrorist threat emanating from there,” he told MPs.

“Reapers are not authorised to use weapons in Syria; that would require further permission.”

Asked why parliament had not been consulted about the use of UK drones to conduct surveillance over Syria, Cameron’s deputy official spokesman said it was because it did not amount to military action.

“We are using these assets to enhance our capability to gather intelligence and protect Britain from threats emanating from Syria,” she said. “The prime minister and government have made clear that we would return to parliament for a separate decision if we were proposing to take military action. This is about intelligence-gathering.

“We’ve also made clear that, if there was a critical national interest at stake or we needed to act swiftly to prevent a humanitarian catastrophe, we would act immediately and explain to parliament afterwards. I think our approach is consistent with that.”

She said the legal case had been set out at the time of the House of Commons debate on action in Iraq, where the prime minister had made clear the government believed military action was allowable in both countries on the basis of “national and collective defence”.

Asked if there had been any discussions with the Syrian government, she said: “We’re clear what the legal case is and that is national and collective self defence … focused on the Isil threat.”

The spokeswoman declined to comment on the purpose of the intelligence to be gathered or whether it would be shared with the US, which is currently leading air strikes in Syria.

“The nature of this approach has been: how do we best protect our national security and how do we work with international allies to do that?” she said.

Pressed on whether it was a step towards inevitable UK military action in Syria, the official stressed that Cameron would return to parliament to seek approval for any such move.

“This is about looking at the nature of the conflict, looking at the nature of the assets we have, and thinking: how can we best deploy those to support our efforts to protect the UK and keep British people safe?”